Most often in January, 27 days.
Highest follower count seen that year: 11,931.
January
Jan 1 00:00–01:00, 19:00–22:00 UTC Jan 2 19:00–23:00 UTC Jan 3 20:00–22:00 UTC Jan 4 20:00–22:00 UTC Jan 6 19:00–23:00 UTC Jan 7 20:00–23:00 UTC Jan 9 19:00–22:00 UTC Jan 10 20:00–23:00 UTC Jan 11 20:00–24:00 UTC Jan 12 19:00–23:00 UTC Jan 13 19:00–23:00 UTC Jan 14 19:00–24:00 UTC Jan 15 19:00–22:00 UTC Jan 16 20:00–22:00 UTC Jan 17 21:00–24:00 UTC Jan 18 00:00–01:00, 21:00–23:00 UTC Jan 20 20:00–24:00 UTC Jan 21 19:00–23:00 UTC Jan 22 20:00–24:00 UTC Jan 23 19:00–20:00, 22:00–23:00 UTC Jan 24 22:00–24:00 UTC Jan 25 22:00–24:00 UTC Jan 26 20:00–23:00 UTC Jan 27 20:00–23:00 UTC Jan 28 20:00–22:00 UTC Jan 30 21:00–23:00 UTC Jan 31 22:00–23:00 UTC
February
Feb 1 00:00–01:00, 20:00–21:00 UTC Feb 3 19:00–21:00 UTC Feb 4 19:00–22:00 UTC Feb 5 20:00–23:00 UTC Feb 6 20:00–22:00 UTC Feb 8 21:00–23:00 UTC Feb 9 20:00–24:00 UTC Feb 10 20:00–22:00 UTC Feb 11 20:00–24:00 UTC Feb 13 20:00–23:00 UTC Feb 14 21:00–24:00 UTC Feb 15 00:00–01:00, 20:00–24:00 UTC Feb 16 20:00–24:00 UTC Feb 17 21:00–24:00 UTC Feb 18 20:00–23:00 UTC Feb 19 20:00–23:00 UTC Feb 20 20:00–23:00 UTC Feb 21 22:00–23:00 UTC Feb 23 19:00–23:00 UTC
March
Mar 14 22:00–23:00 UTC Mar 15 21:00–23:00 UTC Mar 16 21:00–23:00 UTC Mar 17 20:00–22:00 UTC Mar 18 20:00–23:00 UTC Mar 19 20:00–23:00 UTC Mar 20 20:00–21:00 UTC Mar 21 21:00–24:00 UTC Mar 22 21:00–22:00 UTC Mar 23 20:00–23:00 UTC Mar 25 20:00–23:00 UTC Mar 26 22:00–24:00 UTC Mar 28 21:00–24:00 UTC Mar 31 20:00–21:00, 23:00–24:00 UTC
April
Apr 1 00:00–02:00 UTC Apr 2 20:00–24:00 UTC Apr 4 21:00–24:00 UTC Apr 5 00:00–01:00, 20:00–23:00 UTC Apr 6 20:00–22:00 UTC Apr 8 20:00–23:00 UTC Apr 10 20:00–22:00 UTC Apr 11 20:00–23:00 UTC Apr 12 20:00–22:00 UTC Apr 13 20:00–23:00 UTC Apr 15 20:00–23:00 UTC Apr 16 00:00–01:00, 20:00–23:00 UTC Apr 18 21:00–24:00 UTC Apr 19 20:00–24:00 UTC Apr 20 21:00–23:00 UTC Apr 21 00:00–02:00 UTC Apr 23 21:00–23:00 UTC Apr 24 19:00–22:00 UTC Apr 25 21:00–22:00 UTC Apr 26 20:00–22:00 UTC Apr 27 20:00–24:00 UTC Apr 28 00:00–01:00, 20:00–24:00 UTC Apr 29 20:00–23:00 UTC
May
May 1 20:00–22:00 UTC May 2 23:00–24:00 UTC May 3 21:00–24:00 UTC May 4 20:00–23:00 UTC May 5 20:00–23:00 UTC May 6 20:00–24:00 UTC May 7 19:00–21:00 UTC May 8 20:00–22:00 UTC May 10 18:00–21:00 UTC May 12 19:00–23:00 UTC May 13 20:00–23:00 UTC May 14 22:00–23:00 UTC May 15 20:00–23:00 UTC May 16 21:00–24:00 UTC May 19 20:00–24:00 UTC May 20 19:00–22:00 UTC May 21 21:00–24:00 UTC May 22 20:00–24:00 UTC May 23 21:00–24:00 UTC May 24 00:00–01:00, 18:00–24:00 UTC May 25 21:00–24:00 UTC May 27 21:00–24:00 UTC May 28 00:00–01:00, 19:00–23:00 UTC May 29 19:00–24:00 UTC May 30 21:00–23:00 UTC May 31 20:00–23:00 UTC
June
Jun 1 20:00–23:00 UTC Jun 2 19:00–22:00 UTC Jun 3 20:00–23:00 UTC Jun 4 21:00–23:00 UTC Jun 5 20:00–23:00 UTC Jun 6 21:00–24:00 UTC Jun 7 00:00–01:00 UTC Jun 8 20:00–23:00 UTC Jun 9 21:00–23:00 UTC Jun 10 21:00–23:00 UTC Jun 11 21:00–23:00 UTC Jun 15 20:00–23:00 UTC Jun 16 20:00–24:00 UTC Jun 18 20:00–24:00 UTC Jun 19 20:00–22:00 UTC Jun 20 20:00–23:00 UTC Jun 21 20:00–22:00 UTC Jun 22 20:00–23:00 UTC Jun 23 20:00–23:00 UTC Jun 25 20:00–22:00 UTC Jun 26 20:00–23:00 UTC Jun 28 20:00–23:00 UTC Jun 30 19:00–22:00 UTC
July
Jul 1 22:00–24:00 UTC Jul 2 20:00–22:00 UTC Jul 3 19:00–22:00 UTC Jul 4 21:00–24:00 UTC Jul 5 00:00–02:00, 19:00–22:00 UTC Jul 7 21:00–23:00 UTC Jul 8 20:00–21:00 UTC Jul 9 20:00–23:00 UTC Jul 10 21:00–23:00 UTC Jul 11 21:00–23:00 UTC Jul 12 21:00–24:00 UTC Jul 13 21:00–24:00 UTC Jul 14 20:00–23:00 UTC Jul 15 22:00–24:00 UTC Jul 16 19:00–24:00 UTC Jul 17 00:00–01:00, 21:00–22:00 UTC Jul 21 00:00–03:00, 20:00–23:00 UTC Jul 24 01:00–04:00, 20:00–23:00 UTC Jul 25 00:00–02:00, 21:00–23:00 UTC Jul 26 21:00–23:00 UTC